Production of polycyclic aromatic hydrocarbons from high molecular weight paraffins



United States PatentO PRODUCTION OF POLYCYCLIC AROMATIC HY- DROCARBONS FROM HIGH MOLECULAR WEIGHT PARAFFINS No Drawing. Filed Apr. 9, 1958, Ser. No. 727,271

5 Claims. or. 260-6735) This invention relates to a method for converting high molecular weight parafiinic hydrocarbons into polycyclic aromatic hydrocarbons and more particularly this invention relates to a method for catalytically converting paraflinic hydrocarbons containing from 12 to 20 carbon atoms in the molecule into polycyclic aromatic hydrocarbons. 1

For a number of years methods have been known for the conversion of low molecular weight pa'raffinic hydrocarbons containing from 6 to 9 carbon atoms in the molecule into monocyclic aromatic hydrocarbons, i.e.-, benzene and alkyl benzenes. Such dehydrocyclization reactions were accomplished by means of a wide variety of'so-called aromatization catalysts which catalysts, however, were used only for the production of monocyclic aromatic hydrocarbons.

It has now been found, however, that high molecular weight parafiinic hydrocarbons particularly those containing from 12 to 20 carbon atoms in the molecule may be converted into polycyclic aromatic hydrocarbons particularly naphthalene, biphenyl and tricyclic aromatics including the condensed ring tricyclic compounds. This conversion is accomplished by the use of an excess of hy drogen and a catalyst which has as its active components platinum and an active acidic metal oxide which acidic oxide is characterized by its ability to promote the cracking of hydrocarbons and by having exchangeable hydrogen ions in its structure. The conversion is also promoted by the use of certain specific reaction conditions which will be described in detail.

Therefore, it is an object of this invention to provide a method for the conversion of high molecular weight paraffinic hydrocarbons into polycyclic aromatic hydrocarbons.

It is a further object of this invention to provide a method for the catalytic conversion of parafilnic hydrocarbons containing from 12 to 20 carbon atoms in the molecule into polycyclic aromatic hydrocarbons.

It is a further object of this invention to provide a method for the conversion of parafiinic hydrocarbons containing from 12 to 20 carbon atoms in the molecule into polycyclic aromatic hydrocarbons by the use of an excess of hydrogen and a catalyst consisting essentially of platinum associated with an active acidic metal oxide component.

Other objects of this invention will be apparent from the description and claims that follow.

In accordance with this invention, paraffinic hydrocarbons containing from 12 to 20 carbon atoms in the molecule form the preferred hydrocarbon charge material to the process. Examples of such hydrocarbons are dodecane,tridecane, tetradecane, pentadecane, hexadecane (cetane), heptadecane, octadecane, nonadecane and eicosane. In addition to the straight chain paraffins, branched-chain paraflins containing from 12 to 20 carbon atoms in the molecule also form suitable hydrocarbon charge'materials. Hydrocarbons in this molecular weight range are found in certain petroleum distillate fractions such as kerosine, diesel fuel, furnace oil and the like. These distillate fractions also may contain other hydrocarbons such as cycloparaflins and higher molecular weight aromatics; however, the presence of such compounds does not interfere with the production of the polycyclic aromatics from the parafllns. In addition, paraflins in the 12 to 20 carbon atom range produced by the Fischer-Tropsch process form a particularly useful charge material.

If so desired these various petroleum distillate fractions and other fractions may be treated with selective solvents in order to separate highly parafiinic fractions. Such selective solvent treatments are well known to petroleum refiners and have been described widely both in patents and the technical literature. In all instances, however, it has been found that the yield of polycyclic aromatics will be at a maximum when the hydrocarbon fraction contains paraflins having from 12 to 20 carbon atoms in the molecule and therefore it may be necessary to re-fractionate certain distillate fractions in order to remove at least the major proportion of the paraflins below the 12 carbon atoms range and above the 20 carbon atom range. The catalysts which are suitable for the promotion of the conversion of the high molecular weight paraffins into polycyclic aromatics contain two active components, one being platinum and the other being an active acidic metal oxide component. 1 The active acidic metal oxide component is charac; terized by the fact that it has activity for the cracking of hydrocarbons and has exchangeable hydrogen ions in its structure. A particularly suitable example of such a compound-is a commercial silica-alumina cracking catalyst wherein the alumina ranges between 7 and 30 percent by weight with the remainder being silica. Commercial cracking catalysts containing from 12 percent to 25 percent by Weight alumina are particularly preferred. In addition to the silica-alumina, other oxides such as silicamagnesia, silica-thoria,.silica-zirconia, and various combinations of silica with these metallic oxides also may be used, for example, silica-alumina-magnesia, silica-alumina,- thoria and the like. A variety of methods for preparing such components are known and published in the catalytic cracking art, and since this invention is not concerned with any specific method of preparing this component of the catalyst, it is unnecessary to elaborate further on this point. i

A particularly preferred acidic metal oxide component is a commercial silica-alumina cracking catalyst which has had its activity altered. The activity of a cracking catalyst may be measured by a distillate-plus-loss (D+L) scale according to the method of Birkhimer et al., A Bench Scale Test Method for Evaluating Cracking Catalysts, Proceedings of the American Petroleum Institute, Division of Refining, Volume 27 (III), page (1947). According to the Birkhimer et al. D+L activity measure; ment method, it would be possible to have a theoretical maximum D+L of 100; however, in general, the maximum D+L for a fresh synthetic silica-alumina cracking catalyst will range between 90 and 95. While there are a number of other methods of measuring the catalytic cracking activity of synthetic silica-alumina cracking catalysts which have been described in the literature, these methods employ a D+L measure having much lower values for a fresh silica-alumina cracking catalyst, usually of the order of 45 to 65 depending upon the particular test. Accordingly, when there is specified a D-l-L scale having a practicable maximum activity of 90 to the Birkhimer et al. method is being employed. "3 A number of methods for altering the cracking activity of synthetic silica-alumina cracking catalysts have been described; however, the preferred method for the catalyst of the instant invention is treatment of the catalyst with Steam at temperatures offrom 900 F. to 1400 F., at pressures ranging from atmospheric to several hundred pounds per square inch, for 'a period of time sufficient to provide the desired degree of alteration.

It has been found that the acidic metal oxide component most suitable for use in preparing the catalysts for promoting the conversion of the parafiinic hydrocarbons of this invention should have an activity ranging between 35 D+L and 95 D+L, as measured by the/Birkhimer et al. method, the most preferred range, however, being from 45 D+L to '75 D-l-L.

The platinum may be'deposited directly onto the acidic metal oxide component from an aqueous solution of one of its compounds; for example, it may be deposited from an aqueous solution of'chloroplatinic acid, platinous tetraammino chloride, platinous tetrammino hydroxide, platinic hexammino hydroxide, platinic hexammino chloride, chloroplatinous acid, platinicchloride, ammonium chloroplatinate, and similar platinum-containing solutions.

After contacting the acidic metal oxide with the solution of the platinum compound, the excess solution, if any, is removed and the oxide component is dried. The platinum may be reduced to the metallic state by calcination with air at temperaturesranging between 500 F. and 1100" F., or treated with hydrogen at 400 F. to 1000 F., in accordance with well-known methods. The amount of the metallic platinum deposited on the :acidic metal oxide componentmay range from 0.1 percent by weight to 2.5 percent by weight based-onthe weight of the final catalyst.

Alternatively, the platinum may be deposited on an inert carrier. 'Theinert carrier upon which the platinum is deposited may be any one of or a mixture of the commercially available aluminas, such as chi alumina, gamma alumina, eta alumina'and alpha alumina monohydrate. These aluminas are described in the article Thermal Transformationsof Aluminas and Alumina Hydrates, by H. C. Stumpf, A. S. Russell, J. W. Newsome, and C. M. Tucker, in Industrial and Engineering Chemistry, volume 42, page 13928 et seq. (1950). In addition, other inert carriers may beemployed such as magnesium oxide, calcium oxide, titanium oxide, silica gel, fullers earth, kaolin, kieselguhr, diatomaceous earth, bauxite, and naturally occurring adsorbent clays. The various materials which may be used as the inert carrier differ in their ability to absorb or adsorb platinum compounds from solution and, therefore, these carriers are not an equally effective for the purpose of this invention. A particularly effective carrier is alumina having a surface area of from 50 to 400 square meters per gram as measured by the nitrogen adsorption method of Brunnauer, Emmett and Teller found in the Journal of the American Chemical Society, volume 60, pages 309 et seq. (1938).

The deposition of the platinum onthe inert carrier may be made from the same aqueous solutions described above. The amount of platinum deposited upon the inert carrier should be in an amount such that the final catalyst will contain between 0.1 percent and 2.5 percent by weight of platinum based on the weight of the final catalyst. The platinum may be reduced to the metallic state either by air calcination or hydrogen reduction .as has been described.

The-acidic metal oxide component is physically admixed with the inert carrier upon which the platinum has been deposited and the mixture may be pelleted or extruded according to conventional methods to produce masses suitable for use in fixed bed reactors.

In order to pellet the mixture of components, it has been found that for best results, the individual componentsshouldfirst-be reduced to, a particle size of the order not exceeding one millimeter maximum cross sectional 4 dimension, preferably, they .are reduced to a powder capable of being passed through an mesh U.S. Standard Sieve. It has been found that the carrier component upon which the platinum is deposited may range between 5 percent and 75 percent by weight of the final catalyst. The most preferred composite, however, is obtained when the weight of the carrier component is approximately that of the silica-alumina component, i.e., each pomponent constitutes approximately 50=percent of the mixture by weight.

It will be noted that the catalysts which are suitable for this invention contain only two active components, i.e., the platinum and the acidic metal oxide. In those catalysts wherein the-platinumis deposited upon an inert carrier it is necessary that the carrier be admixed intimately with the acidic metal oxide component so that this catalyst is, as far as its activity for promoting the conversion of the high molecular weight parafiins into polycyclic aromatics, an association of platinum with an acidic metal oxide component which is equivalent to catalyst produced by the directdeposition of platinum on the acidic metal oxide component. These catalysts may be used for extended periods of time for the conversion of the paraffins into the polycyclic aromatics, however, when they become sufiiciently deactivated to require regeneration, this may be accomplished by simply burning the catalyst in situ with .a free-oxygen-containing .gas wherein the oxygen content is quite :dilute, of the order of 0.5 to 2.0 percent.

The parafiinic charge-material is passed over the catalyst in the presence of .an excess ofhydrogen, the primary purpose of which is .to prevent or materially reducethe formation of coke or carbonaceous deposits on thecatalyst. The conversion proceeds at temperatures between 600 F. and 1000" F. under pressures between and 750 pounds per square inch with a liquid hourly space velocity of from 0.5 to 10 volumes of liquid charge per volume of catalyst per hour and with a hydrogen to h-ydrocarbon mole ratio of from 2:1 to 40:1. It is preferred, however, to conduct the conversion at tempera tures between 750 F. and 950 F. under pressures between 15() and 500 pounds per square inch with a liquid hourly space velocity of from 1.0 to 6.0 volumes of liquid charge per volume of catalyst per hour and with 22. hydrogen to hydrocarbon mole ratio of from 5:1 to 25:1.

The conversion reaction proceeds with a net production of hydrogen and accordingly the excess hydrogen may be supplied to the process quite simply by recycling a portion of the product hydrogen which may be separated from the reactor effluent.

Since the catalyst employed in the process of this invention has a rather high initial cost it is preferred that the catalyst be used in fixed bed reactors. Moreover, since the reaction is endothermic the catalyst may be contained in a series of reactors with heaters between successive reactors.

The examples which follow will serve to illustrate certain specific embodiments of the invention, but these examples should not be construed as limiting the invention to the specific features set forth therein.

EXAMPLE I A sample of pure standard cetane (hexadecane) was passed over a catalyst prepared by depositing platinum from a chloroplatinic acid solution onto a commercial silica-alumina cracking catalyst (13 percent by weight alumina, 87 percent by weight silica) which had been steam treated to an activity level of 45 D+L as measured by the Birkhimer et al. method. After calcination to reduce the platinum to the metallic state, the catalyst contained 0.45 percent by weight of platinum. The cetane was passed over this catalyst at several temperatures with the other conditions being held constant, i.e., pressure at p.s.i.g., space velocity of 1.0, hydrogen to hydrocarbon mol ratio :1. The results of these experiments are shown in Table I.

Table I Temperature F.) 500 800 900 Product Distribution, Weight Percent, No Loss Basis:

C1 to Paralfins C and heavier Paratfins 1 Cycloparnlfins. Alkyl Benzenes N eph thalenes Tetralins Decalins Biphenyls Tricyolics CAD HUI

EXAMPLE II Another portion of cetane was passed over another portion of the catalyst employed in Example I and under the same conditions of Example I except that a space velocity of 2.0 was employed. The results of these experiments are shown in Table II.

Table II Temperature F.) 800 900 950 Product Distribution, Weight Percent,

No Loss Basis:

C to G5 Paratfins Ca and heavier Paratfins 1 Cycloparafiins Alkylbenzenes Naphthalenes Tetralins 2 .c Deealins Biphenyls. Tricyclics 1 Predominantly unconverted cetane.

2 Includes alkyl derivatives.

These data show that with higher space velocities it is possible to go to higher temperatures although conversion of the high molecular weight paraffins to aromatics is decreased somewhat.

EXAMPLE III A sample of commercial eicosane (95+ percent purity) was diluted with normal heptane to give a mixture of 64 weight percent eicosane and 36 weight percent nheptane. This mixture was passed over another portion of the catalyst employed in Example I and at the same conditions employed in Example I with a reaction temperature of 650 F. The cetane was diluted in order that it could be passed over the catalyst at temperatures below those which would give too much cracking of this high molecular weight paraflin. Since the diluent was much lower in molecular weight than the lowest molecular weight polycyclic aromatic, i.e. naphthalene, it would not contribute to the production of any polycyclic aromatics. It was found that under these conditions there was produced 27.6 weight percent of naphthalenes and a total 6 of 36.8 weight percent of di and polycyclics based on the eicosane in the charge material.

It will be noted that in the examples there are appreciable amounts of alkyl benzenes produced as might be predicted from known aromatization processes. It is quite unexpected, however, that the major reaction is the production of polycyclic aromatics.

I claim:

1. A method for converting solely by dehydrocyclization reactions 12 carbon atom to 20 carbon atom paratfinic hydrocarbons selected from the group consisting of 12 carbon atom to 20 carbon atom straight-chain and 12 carbon atom to 20 carbon atom branched-chain parafiinic hydrocarbons into polycyclic aromatic hydrocarbons which comprises contacting the 12 carbon atom to 20 carbon atom parafiinic hydrocarbons in the presence of excess hydrogen with a catalyst consisting essentially of platinum associated with an acidic metal oxide component at a temperature between 750 F. and 950 F., a pressure between 150 and 500 pounds per square inch, a liquid hourly space velocity between 1.0 and 6.0 volumes of charge per volume of catalyst per hour and a hydrogen to hydrocarbon mol ratio of between 5:1 and 25:1.

2. The method according to claim 1 further characterized in that the catalyst consists essentially of a physical admixture of an inert carrier on which has been deposited 0.1 percent to 2.5 percent by weight of platinum based on the weight of the final catalyst and an active acidic metal oxide component,, the carrier and platinum component being present in an amount ranging between 5 percent and percent by weight of the final catalyst.

3. The method according to claim 2 further characterized in that the inert carrier is alumina and the active acidic metal oxide component is a silica-alumina cracking component having an alumina content of from 7 percent to 30 percent by Weight and having a catalytic cracking activity within a range between 35 and 95 as compared with a theoretical maximum catalytic cracking activity of 100 and a practicable maximum catalytic cracking activity of between and on a distillatepllus-loss scale for the measurement of the catalytic cracking activity of a cracking catalyst.

4. The method according to claim 1 further characterized in that the platinum ranges between 0.1 percent and 2.5 percent by weight based on the weight of the final catalyst and the active acidic metal oxide component is a silica alumina cracking component having an alumina content of from 7 percent to 30 percent by weight and having a catalytic cracking activity within a range between 35 and 95 as compared with a theoretical maximum catalytic cracking activity of and a practicable maximum catalytic cracking activity of between 90 and 95 on a distillate-plus-loss scale for the measurement of the catalytic cracking activity of a cracking catalyst.

5. The method according to claim 4 wherein the silica-alumina component has a catalytic cracking activity within a range between 45 and 75 on the distillate-plusloss scale.

References Cited in the file of this patent UNITED STATES PATENTS 2,475,977 Meier July 12, 1949 2,651,598 Ciapetta Sept. 8, 1953 2,763,623 Haensel Sept. 18, 1956 FOREIGN PATENTS 779,497 Great Britain July 24, 1957 

1. A METHOD FOR COVERING SOLELY BY DEHYDROCYCLIZATION REACTIONS 12 CARBON ATOM TO 20 CARBON ATOM PARAFFINIC HYDROCARBONS SELECTED FROM THE GROUP CONSISTING ING OF 12 CARBON ATOM TO 20 CARBON ATOM STRAIGHT-CHAIN AND 12 CARBON ATOM TO 20 CARBON ATOM BRANCHED-CHAIN PARAFFINIC HYDROCARBONS INTO POLYCYCLIC AROMATIC HYDROCARBONS WHICH COMPRISES CONTACTING THE 12 CARBON ATOM TO 20 CARBON ATOM PARAFFINIC HYDROCARBONS IN THE PRESENCE OF EXCESS HYDROGEN WITH A CATALYST CONSISTING ESSENTIALLY OF PLATINUM ASSOCIATED WITH AN ACIDIC METAL OXIDE COMPONENT AT A TEMPERATURE BETWEEN 750*F. AND 950*F., A PRESSURE BETWEEN 150 AND 500 POUNDS PER SQUARE INCH, A LIQUID HOURLY SPACE VELOCITY BETWEEN 1.0 AND 6.0 VOLUMES OF CHARGE PER VOLUME OF CATALYST PER HOUR AND A HYDROGEN TO HYDROCARBON MOL RATIO OF BETWEEN 5:1 AND 25:1. 